Power control of the transmission signal is a critical performance and efficiency aspect in wireless communication systems and associated networks such as Global System Mobile (GSM), Enhanced Data rate for GSM Evolution (EDGE), Wideband Code Division Multiple Access (WCDMA), High Speed Downlink Packet Access (HSDPA) systems and the like. In order to meet rigid specifications for transmission in such environments including transmission power vs. time masks, frequency domain transmission power emission masks, and the like, particularly over a variety of temperature ranges, power supply voltage ranges, and the like, precise transmit power control must be achieved, most often with closed loop power control.
A target power level associated with a transmission may be achieved, given transmit RF/IF path gain variations, using closed loop control. Closed loop control is often used to achieve the Power versus Time masks and transient Adjacent Channel Power (ACP) levels in accordance with the relevant specifications, as well as to perform power amplifier (PA) load switching to improve PA efficiency. Using the basic elements of a modern closed loop power control system including one or more of baseband gain control, Intermediate Frequency (IF) gain control, Radio Frequency (RF) gain control, and the like, the transmit power gain of RF stages such as the RF Voltage Controlled Amplifier (VCA) and the RF PA can be adjusted to meet demands. An RF power detector and an A/D converter facilitate digital closed loop power control where a detected digital signal power level is compared to a pre-programmed reference signal to generate an error signal. A loop filter controls the loop dynamics of the control system by filtering the error signal and providing a control output which is converted and used to control the transmit power level through, for example, the VCA and PA power control stages.
Limitations in conventional transmit power control systems arise where, for example, the transmitted power has a higher dynamic range than the A/D converter. As a result, the closed loop power control range is limited leading to performance degradation such as loss of power accuracy, failure to meet Power versus Time masks, unacceptable transient ACP, and the like. Further, PA efficiency can be reduced due to current drain resulting from the loss of load switching capability in the PA across the lost converter range. Further limitations occur during transmit power transitions associated with switching from a power level associated with, for example, a first slot, to a power level associated with a second slot. The transitions require a ramp-up or ramp-down depending on the next power level and poor power control during such intervals can cause disturbances in the loop bandwidth ultimately increasing convergence or settling time for new gain levels. Transmit power transients and the like during ramp or transition intervals may further cause a transmitter to exceed power masks and result in instability at least for a period of time until a post transition gain level settles.